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Agate is the ayurvedic birthstone for the month of May and the mystical birthstone for the month of September. 

The name is derived from its occurrence at the Achates River in southwestern Sicily and received its name from the Ancient Greek philosopher and naturalist Theophrastus, who found the gemstone on the banks of the Achates River (Dirillo river), Acate, Ragusa Province, Sicily, Italy.

Agate is a distinctly banded fibrous formation of common semi-precious chalcedony (Formula: SiO2). Agate is the compact, microcrystalline variety of quartz. It is made of fibrous length-fast chalcedony, sometimes with layers of quartzine (length-slow chalcedony) fibers (Michel-Lévy and Munier-Chalmas 1892; Correns and Nagelschmidt, 1933; Bernauer 1927; Braitsch 1957; Frondel, 1978; Flörke et al. 1991; Graetsch 1994).
Most agates form in cavities in ancient lavas or other extrusive igneous rocks. They are characterized for the most part by color bands in a concentric form, and less often by mosslike inclusions (moss agate). Other names often precede the word agate and indicate either its visual characteristics or its place of origin. Fortification agate is a type of banded agate with angularly arranged bands that resemble an aerial view of an ancient fortress. There are several types of fortification agate. Brazilian agate is a fortification agate with banding in angled concentric circles like the fortifications of a castle. Botswana agate from Africa had beautiful dark and light banding. Mexican lace--sometimes called "crazy lace"--is a multicolored fortification agate with highly convoluted layering. Blue lace agate from South Africa--one of the most common kinds for sale today--is a delicate light blue with a fine interlayering of colorless agate. Fire agate has inclusions of reddish to brown hematite that give an internal iridescence to polished stones. There are a number of varieties of chalcedony that are called "agate" that do not match the definition given above. Good examples are "feather agates" and "fire agates". These are listed as varieties of chalcedony, not as varieties of agate. Because the colors and patterns found in agates are so varied and so characteristic for the respective localities, there is a confusingly large number of ever-changing varietal and trade names. Roger Pabian's "Agate Lexicon" at UNL is a good source: Agates Lexicon




In thin slices of agate, the fibers are sometimes visible in transmitted light and may cause interesting optical effects (see Iris Agate). The banding in agate is based on periodic changes in the translucency of the agate substance. Layers appear darker when they are more translucent (this may appear reversed in transmitted light). This effect may be accompanied and amplified by changes in the color of neighboring layers, due to other co-precipitated minerals and occur in shades of white, yellow, gray, pale blue, brown, pink, red, or black. Much of the sliced agate offered on the market in particularly bright colors are dyed or stained to enhance the natural color, an easy process because the individual crystallites in the fibers are not tightly interlocked, agate is slightly porous (e.g., Monroe, 1964), and can be dyed easily.  In old agates that have been subject to weathering and chemical alteration, the differences in translucency may disappear, and such specimens may turn almost opaque.
Banded agate is produced by a series of processes that take place in cavities in solidified lava. As the lava cools, steam and other gasses form bubbles in the liquid rock that are preserved as the lava hardens, forming cavities. Long after the rock has solidified, silica-bearing water solutions penetrate into a bubble and coagulate to form a layer of silica gel. This sequence is repeated until the hollow is filled (sometimes occurring inside of quartz geodes). Some of the layers will have picked up traces of iron or other soluble material to give the bands remaining undisturbed. Note: Agate is not simply "banded chalcedony." There are other types of chalcedony that are banded that do not match the description above; banded flint, for example. Two characteristic types of banding can be distinguished in agates (e.g. Graetsch 1994):

1. Wall-lining Banding. (The characteristic bands usually follow the outline of the cavity in which the mineral has formed.) The individual bands run perpendicular to the orientation and growth direction of the chalcedony fibers. Since the chalcedony fibers grow from the walls to the interior of a cavity, a concentric, onion-like pattern develops. The changes in translucency reflect periodic changes of crystallite sizes and repetitive nucleation of new fibers at the growth front (Taijing and Sunagawa 1994; Cady et al 1998), as well as chemical composition (Frondel 1978; Heaney and Davis 1995). In addition to the visible bands, there are compositional bands of varying trace element and hydroxyl concentrations on the micrometer scale (Frondel 1978, 1985).
Note that this type of banding is not restricted to walls of geodes: similar-looking patterns of banding will develop around other structures that grew into the cavity, like crystals or moss-like inclusions.

2. Horizontal Banding (also called Uruguay-type banding). This type of banding is less common and usually accompanied by wall-lining banding. The banding consists of fine, irregularly spaced layers of small chalcedony spherulites and sometimes quartz crystals that precipitated in the cavity. Horizontal bands can serve as spirit levels to determine the original orientation of the specimen in the host rock. When the difference in translucency or color between the layers is pronounced, agates with horizontal banding can be used for cutting cameos and engravings.

Agates have been worked since prehistoric times and were among the world's first lapidary materials. Today, it is usually cut as thin slabs, or polished ornaments broaches, or cut as cabochons for pendants. But the decorative uses of this hardstone have origins as far back as the Bronze Age. Remnants of agate crystal have been found in Knossos, Crete, signifying a role in the Minoan culture. Agate stone was also found along the Nahe River in modern-day Germany during the reign of Julius Caesar. Using running water-powered facilities, the Romans cut these agate stones into various shapes for various uses until the resources depleted in the 1800s. Renaissance artisans found many applications for agate, too; collecting bowls made of agate were especially popular.
Metaphysical properties have been attributed to agate for thousands of years. Some believe agate has the ability to dispel dysfunctional or problematic energies and dreams, while others believe it balances one’s physical, emotional, and spiritual auras. Agate is believed to discern truth, accept circumstances, and is a powerful emotional healer. Legend says that Agate helps to prevent insomnia and insure pleasant dreams, to enhance personal courage, and protect one against danger. improves memory and concentration, increases stamina, and encourages honesty. Grey agates were worn by ancient Egyptians to prevent and remedy stiff necks. In Persia and parts of Asia, the gemstone was believed to reduce fevers.

Agate has long been beloved by royalty, as well. In particular, Queen Victoria made Scottish agate a staple in her wardrobe. This fashion choice led to an explosion in the popularity of agate jewelry during her reign. In fact, Victorian-era jewelry with agate is highly sought after by today’s collectors and connoisseurs.

Agate is found worldwide, but Brazil, Botswana, South Africa, Mexico, Egypt, China, and Scotland are prolific sources and produce interesting varieties. Oregon, Washington, Idaho, and Montana are popularly abundant sources in the US.